It is now common practice to modify television material to account for different display aspect ratios (image width to height ratios). Aspect ratios of 4:3 and 16:9 are widely used and conversion of images originated in one of these ratios for display at the other ratio is often needed. One method of increasing the width of a 4:3 image to fill a 16:9 display is to apply horizontal magnification to regions adjacent to the left and right edges of the image, whilst leaving the central image region unchanged. The horizontal magnification factor in the image-edge regions may vary smoothly between a higher value close to the image edges and a lower value at the junctions between the edge regions and the central region. The image is thus differently-rescaled in different image regions.
It is helpful to be able to detect automatically that this type of ‘non-uniform aspect ratio conversion’ has been applied to an image. Such a detector can form part of a broadcast-chain monitoring or quality-assurance system.